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Moving decimal spaces?

Blueshark1 (29)
For this project we are supposed to have a table of decimals produced from our code. The code mainly works fine, it's that I do not know how to move the decimal space for the list of numbers. For example, I am trying to get -2.000000+01 and -3.6945283e-01, but instead I get -20.000000 -0.369453 as shown in the output section below. Any advice?

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#include<iostream>		//Required for cin, cout
#include<cmath>			//Required for sin()
#include<fstream>		// to build data file to be used by Excel to plot
#include <string>		// for using filename and
using namespace std;
double ROBBINS (double x);	//Function Prototype
int
main ()
{
  double a, b, x_incr, new_x;	// Declare objects in main()
  int Number, loop;
  string filename;
  ofstream fout;		// declare object for sending data to the output file
  cout <<
    "enter the name of the data file to send data used for plotting, from this program\n";
  cin >> filename;
  fout.open (filename);		// open file for inputing data from this program
  cout << "Enter endpoint range a and b (a<b): of the function to be plotted \n";	// k<=20
  cin >> a >> b;
  cout <<
    "Enter the number of points you want over the range of 'a' to 'b'\n";
  cin >> Number;
  x_incr = (b - a) / Number;	// increment between the points based on number wanted
  loop = int (b - a) / x_incr;	// loop gives the for loop repetitions equal to number points wanted
  cout << loop;			// check that shows loop and Number are the same
  cout.setf (ios::fixed);
  cout.precision (6);		// Set Formats
  cout << "x and ROBBINSX \n";	// table of sinc(x) values call to sinc()
  for (int k = 0; k <= loop; k++)
    {
      new_x = a + k * x_incr;
      cout << new_x << " " << ROBBINS (new_x) << endl;	//echo vales to screen, call to sinc() here!
      fout << new_x << " " << ROBBINS (new_x) << endl;	// building data file for excel plotting
    }
  return 0;			// Exit program.
  fout.close ();
}				// end main unction used on the next slide 

double ROBBINS (double x)		/* DEMO This function evaluates the sinc function */
{
  if (x == 0)			// x here picks up the values of x in main()
    {
      return 1;			// but this x has its own memory and does not
    }				// interfere with the values of x in main()
  else
    {
      return -exp(fabs(x)/10) /fabs(x);
    }
    /* OUTPUT
    enter the name of the data file to send data used for plotting, from this program
MonaeFile
Enter endpoint range a and b (a<b): of the function to be plotted 
-20 20
Enter the number of points you want over the range of 'a' to 'b'
100x and ROBBINSX 
-20.000000 -0.369453
-19.600000 -0.362211
-19.200000 -0.355258
-18.800000 -0.348591
-18.400000 -0.342203
-18.000000 -0.336092
-17.600000 -0.330252
-17.200000 -0.324682
-16.800000 -0.319378
-16.400000 -0.314340
-16.000000 -0.309565
-15.600000 -0.305053
-15.200000 -0.300804
-14.800000 -0.296821
-14.400000 -0.293104
-14.000000 -0.289657
-13.600000 -0.286485
-13.200000 -0.283593
-12.800000 -0.280987
-12.400000 -0.278679
-12.000000 -0.276676
-11.600000 -0.274994
-11.200000 -0.273648
-10.800000 -0.272656
-10.400000 -0.272040
-10.000000 -0.271828
-9.600000 -0.272052
-9.200000 -0.272749
-8.800000 -0.273966
-8.400000 -0.275758
-8.000000 -0.278193
-7.600000 -0.281352
-7.200000 -0.285338
-6.800000 -0.290276
-6.400000 -0.296325
-6.000000 -0.303686
-5.600000 -0.312620
-5.200000 -0.323467
-4.800000 -0.336682
-4.400000 -0.352888
-4.000000 -0.372956
-3.600000 -0.398147
-3.200000 -0.430352
-2.800000 -0.472546
-2.400000 -0.529687
-2.000000 -0.610701
-1.600000 -0.733444
-1.200000 -0.939581
-0.800000 -1.354109
-0.400000 -2.602027
0.000000 1.000000
0.400000 -2.602027
0.800000 -1.354109
1.200000 -0.939581
1.600000 -0.733444
2.000000 -0.610701
2.400000 -0.529687
2.800000 -0.472546
3.200000 -0.430352
3.600000 -0.398147
4.000000 -0.372956
4.400000 -0.352888
4.800000 -0.336682
5.200000 -0.323467
5.600000 -0.312620
6.000000 -0.303686
6.400000 -0.296325
6.800000 -0.290276
7.200000 -0.285338
7.600000 -0.281352
8.000000 -0.278193
8.400000 -0.275758
8.800000 -0.273966
9.200000 -0.272749
9.600000 -0.272052
10.000000 -0.271828
10.400000 -0.272040
10.800000 -0.272656
11.200000 -0.273648
11.600000 -0.274994
12.000000 -0.276676
12.400000 -0.278679
12.800000 -0.280987
13.200000 -0.283593
13.600000 -0.286485
14.000000 -0.289657
14.400000 -0.293104
14.800000 -0.296821
15.200000 -0.300804
15.600000 -0.305053
16.000000 -0.309565
16.400000 -0.314340
16.800000 -0.319378
17.200000 -0.324682
17.600000 -0.330252
18.000000 -0.336092
18.400000 -0.342203
18.800000 -0.348591
19.200000 -0.355258
19.600000 -0.362211
20.000000 -0.369453
*/
}				// end of function
Last edited on
dhayden (5798)
Perhaps line 26 should be cout.setf (ios::scientific);
deleted account xyzzy (5768)
Just for gits and shiggles... if your compiler supports C++20 you could get a nice looking table output without too much hassle using std::format:
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#include <iostream>
#include <format>
#include <vector>

int main()
{
   // fill a vector with by-rote data to display
   std::vector vec { -20.000000, -0.369453,
                     -19.600000, -0.362211,
                     -19.200000, -0.355258,
                     -18.800000, -0.348591,
                     -18.400000, -0.342203,
                     -18.000000, -0.336092,
                     -17.600000, -0.330252,
                     -17.200000, -0.324682,
                     -16.800000, -0.319378,
                     -16.400000, -0.314340,
                     -16.000000, -0.309565,
                     -15.600000, -0.305053,
                     -15.200000, -0.300804,
                     -14.800000, -0.296821 };

   for (size_t i { }; i < vec.size(); i += 2)
   {
      std::cout << std::format("{:<15.5E}{:<15.5e}\n", vec[i], vec[i + 1]);
   }
}
-2.00000E+01   -3.69453e-01
-1.96000E+01   -3.62211e-01
-1.92000E+01   -3.55258e-01
-1.88000E+01   -3.48591e-01
-1.84000E+01   -3.42203e-01
-1.80000E+01   -3.36092e-01
-1.76000E+01   -3.30252e-01
-1.72000E+01   -3.24682e-01
-1.68000E+01   -3.19378e-01
-1.64000E+01   -3.14340e-01
-1.60000E+01   -3.09565e-01
-1.56000E+01   -3.05053e-01
-1.52000E+01   -3.00804e-01
-1.48000E+01   -2.96821e-01

The format specifier is left-justify (<), a total block of 15 characters with 5 decimal places in scientific format.
jonnin (11441)
slight picking..
abs works on doubles in c++. fabs is dated (or C code).
lastchance (6980)
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#include <iostream>
#include <iomanip>
#include <cmath>
#include <fstream>
#include <string>
using namespace std;

double sinc( double x ) { return abs( x ) < 1e-30 ? 1.0 : sin( x ) / x; }

int main()
{
   string filename;
   cout << "Enter filename for output: ";   cin >> filename;
   ofstream fout( filename );
   
   double a, b;
   int n;
   cout << "Enter a, b, n for n points in [a,b]: ";   cin >> a >> b >> n;
   double dx = ( b - a ) / ( n - 1 );

   #define FMT << scientific << setprecision( 6 ) << setw( 18 ) <<
   cout FMT "x" FMT "sinc(x)" << '\n'; 
   for ( int k = 0; k < n; k++ )
   {
      double x = a + k * dx, y = sinc( x );
      cout FMT x FMT y << '\n';
      fout FMT x FMT y << '\n';
   }
}



Plot with, e.g.,
gnuplot -persist -e "p 'results.txt' w l"


Enter filename for output: results.txt
Enter a, b, n for n points in [a,b]: -20 20 81
                 x           sinc(x)
     -2.000000e+01      4.564726e-02
     -1.950000e+01      3.105333e-02
     -1.900000e+01      7.888274e-03
     -1.850000e+01     -1.851247e-02
     -1.800000e+01     -4.172151e-02
     -1.750000e+01     -5.575006e-02
     -1.700000e+01     -5.655279e-02
     -1.650000e+01     -4.313851e-02
     -1.600000e+01     -1.799396e-02
     -1.550000e+01      1.332048e-02
     -1.500000e+01      4.335252e-02
     -1.450000e+01      6.447552e-02
     -1.400000e+01      7.075767e-02
     -1.350000e+01      5.953959e-02
     -1.300000e+01      3.232054e-02
     -1.250000e+01     -5.305752e-03
     -1.200000e+01     -4.471441e-02
     -1.150000e+01     -7.612628e-02
     -1.100000e+01     -9.090820e-02
     -1.050000e+01     -8.378055e-02
     -1.000000e+01     -5.440211e-02
     -9.500000e+00     -7.910644e-03
     -9.000000e+00      4.579094e-02
     -8.500000e+00      9.393966e-02
     -8.000000e+00      1.236698e-01
     -7.500000e+00      1.250667e-01
     -7.000000e+00      9.385523e-02
     -6.500000e+00      3.309538e-02
     -6.000000e+00     -4.656925e-02
     -5.500000e+00     -1.282801e-01
     -5.000000e+00     -1.917849e-01
     -4.500000e+00     -2.172289e-01
     -4.000000e+00     -1.892006e-01
     -3.500000e+00     -1.002238e-01
     -3.000000e+00      4.704000e-02
     -2.500000e+00      2.393889e-01
     -2.000000e+00      4.546487e-01
     -1.500000e+00      6.649967e-01
     -1.000000e+00      8.414710e-01
     -5.000000e-01      9.588511e-01
      0.000000e+00      1.000000e+00
      5.000000e-01      9.588511e-01
      1.000000e+00      8.414710e-01
      1.500000e+00      6.649967e-01
      2.000000e+00      4.546487e-01
      2.500000e+00      2.393889e-01
      3.000000e+00      4.704000e-02
      3.500000e+00     -1.002238e-01
      4.000000e+00     -1.892006e-01
      4.500000e+00     -2.172289e-01
      5.000000e+00     -1.917849e-01
      5.500000e+00     -1.282801e-01
      6.000000e+00     -4.656925e-02
      6.500000e+00      3.309538e-02
      7.000000e+00      9.385523e-02
      7.500000e+00      1.250667e-01
      8.000000e+00      1.236698e-01
      8.500000e+00      9.393966e-02
      9.000000e+00      4.579094e-02
      9.500000e+00     -7.910644e-03
      1.000000e+01     -5.440211e-02
      1.050000e+01     -8.378055e-02
      1.100000e+01     -9.090820e-02
      1.150000e+01     -7.612628e-02
      1.200000e+01     -4.471441e-02
      1.250000e+01     -5.305752e-03
      1.300000e+01      3.232054e-02
      1.350000e+01      5.953959e-02
      1.400000e+01      7.075767e-02
      1.450000e+01      6.447552e-02
      1.500000e+01      4.335252e-02
      1.550000e+01      1.332048e-02
      1.600000e+01     -1.799396e-02
      1.650000e+01     -4.313851e-02
      1.700000e+01     -5.655279e-02
      1.750000e+01     -5.575006e-02
      1.800000e+01     -4.172151e-02
      1.850000e+01     -1.851247e-02
      1.900000e+01      7.888274e-03
      1.950000e+01      3.105333e-02
      2.000000e+01      4.564726e-02


Last edited on
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